It is well known to provide a spectrophotometer for the quantitative and/or qualitative determination of substances of interest in a test sample material, particularly a solution. Such a spectrophotometer detects electro-magnetic energy, typically optical energy, at one or more defined wavelengths after its interaction with a test sample retained in a sample holder, such as a cell or cuvette. This spectrophotometer device may be configured to operate in one or more of the well known transmission, reflectance or transflectance modes and may, for example, comprise a dispersion element monochromator or may, for example, be configured as an interferometer, such as a Fourier Transform interferometer.
A sample is conventionally poured into the cell or cuvette. When a spectrophotometric measurement is to be performed over a short path length, such as for samples having large absorption for the wavelength used, the sample may need to be pumped into the cell or cuvette. Where a short path length is used, the path length need to be stable and accurately controlled, since a small difference in path length will now represent a greater percentage change in the path length and will therefore greatly affect the measurement results.
It is also known from for example U.S. Pat. No. 5,602,647 to provide an optical spectrophotometer in which a sample holder has a variable internal optical path length. In this spectrophotometer the path length is varied in order to optimize the intensity of a particular wavelength detected. The spectrophotometer is configured to make the quantitative and/or qualitative determination based on the intensity of transmitted optical radiation and the values of the optical path length at peak intensity positions. In this spectrophotometer, it is very important that the variable path length can be accurately controlled. An incorrect path length used for quantitative determination will result in an incorrect result from the measurements. Thus, the variation of the optical path length may introduce faults in the results, if the path length is not accurately controlled.
U.S. Pat. No. 6,628,382 discloses another apparatus for spectrophotometry on extremely small liquid samples. The apparatus comprises two anvil surfaces. One of the surfaces may be swung clear of the other such that the surfaces may be easily cleaned and a sample may be easily applied. A liquid drop is placed on a surface, and the other surface is brought in contact with the drop. Thereafter, the surfaces are drawn apart so that the sample is pulled into a column. In this position, the spectrophotometric measurement is performed. Two measurements may alternatively be performed at two different path lengths. This is suitable where the accuracy of the path difference for a small path difference can be better determined than the absolute full optical path. According to U.S. Pat. No. 6,628,382, the optical path length must still be very accurately controlled.
GB 796,745 discloses a variable path-length absorption cuvette for liquids having a sample contacting surface and a sample receiving surface that are relatively slidable in order to vary an optical path-length between the two surfaces. The sliding mount is of a relatively complex and hence expensive construction, arranged to ensure that the two surfaces remain accurately parallel to one another as the path-length is varied.